There are annually forty-eight million patients suffering from entamebiasis around the world, in particular, in tropical developing countries, and the estimated number of deaths per year is about seventy thousand. In Japan, entamebiasis is designated as a category 5 infectious disease, and 500 to 600 cases have been reported annually.
Overview of Entamebiasis
Entamoeba histolytica is an anaerobic or microaerophilic protozoan that is parasitic in the large intestinal tract. Infection to human is caused by ingesting a food or water contaminated with cysts. Excystation of the cysts occurs in the small intestine, and the cysts become trophozoites and reach the large intestine to form ulcerating pathogens on the mucosal surfaces of the large intestine including the rectum, sigmoid colon, cecum, and ascending colon. Not all infected subjects exhibit symptoms of the disease, and only 5 to 10% of the infected subjects are considered to exhibit symptoms of the disease. Subjects exhibiting symptoms of the disease show dysenteric symptoms such as mucous and bloody stool, diarrhea, tenesmus, aerenterectasia, and lower abdominal pain during defecation. In typical cases, patients pass mucous and bloody stool, and exacerbation and remission are repeated at intervals of several days to several weeks. In exacerbation cases, enterobrosia is caused. Extraintestinal lesions are observed in about 5% of patients exhibiting colitis symptoms. In particular, abscesses are formed in organs or tissues such as liver, lung, brain, and skin. Of those, liver abscesses are most frequently caused and are accompanied by fever with temperatures of 38 to 40° C., hypochondrial pain, nausea, vomiting, weight loss, night sweat, or generalized fatigability. When abscesses burst, lesions are formed in the peritoneum, pleura, and epicardium, resulting in severe symptoms. Trophozoites are encysted in the large intestine and discharged to feces, which are orally ingested by another person, thereby infection is established.
Problems of Entamebiasis Treatment
In general, treatment of entamebiasis is performed by oral administration of metronidazole (product name, Flagyl), which has a high therapeutic effect on symptomatic persons. However, metronidazole is absorbed well from the digestive tract but has a low killing effect on cysts in the intestinal tract, and it is not effective for group treatment of cyst carriers. In order to treat the carriers, metronidazole is used together with diloxanide furoate, which is absorbed from the digestive tract at a low efficiency. However, the killing effect on cysts is insufficient in some cases. Another problem of metronidazole is to readily cause in vitro resistance. In view of the actual state of appearance of resistant strains in another protozoan such as Plasmodium, it is only a matter of time before an Entamoeba histolytica strain resistant to metronidazole appears. In the case of Trichomonad that is an anaerobic protozoan like Entamoeba histolytica, a clinical strain resistant to metronidazole has been reported. Therefore, it is necessary to synthesize a novel compound having the effect of killing Entamoeba histolytica. 
Coombs et al. and we have reported that trifluoromethionine (compound A shown below) has pesticidal activity on anaerobic protozoans such as Entamoeba histolytica and Trichomonas vaginalis (Non-patent Document 1 or 2). The pesticidal activity of trifluoromethionine on the protozoans depends on an enzyme that is present specifically in protozoan and called methionine gamma-lyase. Methionine gamma-lyase is an enzyme involved in decomposition of sulfur-containing amino acids and is not present in mammals, and therefore, the compound has selective effects for protozoans.

Non-patent Document 1: G. H. Coombs, J. C. Mottram, Antimicrob. Agents Chemother 2001, 45, 1743
Non-patent Document 2: M. Tokoro et al., J. Biol. Chem. 2003, 278, 42717-42727